The present invention relates to a geodetic apparatus for performing measurements with respect to a target. Such a geodetic apparatus may be a tachymeter, a theodolite, a total station or a surveyor's level, or a combination thereof, for example.
The target may be a measuring rod such as a ranging-pole or a leveling staff, for example. The ranging-pole allows detection of the same in a field of view of the geodetic apparatus and thus mainly serves for angle and distance measurement. The leveling staff additionally has a pattern (similar to a digital chain) thereon that can be read out by the geodetic apparatus to indicate a relative height between the leveling staff and the geodetic apparatus (provided a sighting axis of the apparatus is arranged in a truly horizontal plane). To facilitate measurement, the measuring rod may be provided with a reflecting surface or a pattern of two or more portions having different reflection capabilities. Further, the measuring rod may be provided with an additional reflector. Furthermore, the measuring rod may be painted by using a typical color or pattern of two or more different colors. Alternatively, the target may even be a prism, an active target emitting a signal, an arbitrary object, or a landmark, for example. Usage of a prism as target provides a high accuracy when used in combination with an electronic distance measuring device (EDM), for example. Usage of an active target may facilitate tracking of the target.
Such a geodetic apparatus is used in surveying and mapping and construction engineering, for example. It is suitable in any field requiring at least one of distance measurement, position measurement, goniometry (measurement of angles) and measurement of a relative height difference with respect to the target. Moreover, it may be used to transfer geometric points from e.g. a technical drawing to the “real world”-environment (e.g. when setting out boundary marks).
When using a geodetic apparatus in combination with a target it is necessary to locate the target in a field of view of the geodetic apparatus. This can be performed either by adjusting the orientation of the geodetic apparatus or by adjusting the position of the target, depending on whether the actual position of the target or a preset position stored in the geodetic apparatus is to be the point of reference (even called geodetic point) for the measurement.
The field of view of the geodetic apparatus depends on the measurement unit of the geodetic apparatus. In case of an optical geodetic apparatus, the field of view depends on the optics of the geodetic apparatus and can frequently be altered from a near-field measurement to a far-field measurement by using adjustable lenses (such as liquid lenses or a zoom lens arrangement). In the present document, the field of view is the spatial area in which the geodetic apparatus is capable of performing measurements with respect to the target without changing the orientation of the geodetic apparatus. Alignment between the target and the geodetic apparatus is achieved when the target is located in a preset position of the field of view of the geodetic apparatus. The straight line between the preset position and the apparatus is called sighting axis. This sighting axis is frequently provided in the middle of the field of view of the geodetic apparatus. A reticule in a display or ocular of the apparatus often visually symbolizes this sighting axis. The sighting axis may be predefined by optics of the apparatus or even be defined dynamically in dependency to the location of the preset position in the field of view of the apparatus.
The present application deals with the case where the preferences of the geodetic apparatus regarding at least one of orientation, distance and height of the target relative to the apparatus are the point of reference for the measurement. Thus, the position of the target has to be adjusted in dependency on preferences given by the geodetic apparatus by moving the target until the target is located at a preset position. At the beginning, the target usually is separated from the sighting axis (even called sighting direction) of the apparatus and thus the preset position in various directions by different distances. This distance relative to the sighting axis, and especially to the preset position, frequently is called offset.
Traditionally, such an adjustment of the position of the target to the geodetic apparatus requires the presence of two users interacting with one another. A first user alters the position of the target while the second user operates the geodetic apparatus to check whether the target is located at the preset position relative to the geodetic apparatus. The first user advises the second user to alter the position of the target until the target is positioned at the present position (with respect to at least one of angle, distance and height) with respect to the geodetic apparatus.
This traditional approach has the disadvantage that the presence of at least two interacting people is necessary to adjust the position of the target with respect to the geodetic apparatus. This is frequently difficult as a distance between the user operating the geodetic apparatus and the user carrying the target might be very long. It would be extremely time consuming for one user to perform such adjustments of the position of the target on their own, as the single user in turn would have to alter the position of the target and operate the geodetic apparatus.
To overcome this problem, a geodetic apparatus provided with auxiliary means for facilitating the setting out of boundaries is described in the U.S. Pat. No. 4,560,270. This apparatus comprises a one-way sound transmitting facility, by means of which audible instructions can be sent from the measuring apparatus to a user carrying a measuring rod comprising a prism. This facility enables the operator of the apparatus to verbally direct the user carrying the measuring rod to the desired boundary mark. Furthermore, this apparatus is also provided with a line-sighting instrument mounted in a fixed position in relation to the apparatus but not connected electronically to it. The line-sighting instrument continuously emits two slightly diverging light beams of mutually different character. The two light beams overlap in a narrow central zone. The user then moves the measuring rod to a point on the plot within the central zone at which the two light beams transmitted from the direction indicating unit of the instrument overlap and at which the prism is in alignment with the sighting axis of the measuring apparatus.
It is crucial with the geodetic apparatus comprising the line-sighting instrument known from the prior art that the narrow central zone of the line-sighting instrument perfectly aligned with a sighting axis and measuring path, respectively, of the geodetic apparatus. Moreover, the line-sighting instrument has to guarantee that the two light beams overlap in a very narrow central zone, both in case of a near-field measurement and of a far-field measurement. Even at the maximum working distance between the prism and the apparatus the central zone in which the two different light beams overlap may not be significantly larger than the measuring path of the apparatus. In consequence, the optics of the line-sighting instrument has to be both of sufficient quality and has to be aligned perfectly with the measuring path of the apparatus. This results in considerable manufacturing costs for the line-sighting instrument. Furthermore, there is a substantial risk that the line-sighting instrument becomes misaligned during use and thus has to be justified again. Further, due to the narrow central zone, a user located on either side of the central zone is only capable to see either one of both light beams. The light-sighting instrument must emit both light beams constantly.
A geodetic instrument using the above line-sighting instrument is known from U.S. Pat. No. 5,051,934. According to this prior art, a distance measuring instrument has a prism tracking facility, which holds the instrument automatically in constant alignment with a prism carried on a setting-out rod, when the prism is located in the path of a measuring beam emitted from an electronic distance meter EDM incorporated in the measuring instrument. Thus, the prism is constantly maintained in a measurement axis of the EDM by automatically rotating the instrument about a vertical and a horizontal axis. The EDM takes continuous measurements against the prism carried on the setting-out rod, as the prism bearer carries around the rod and prism. The instrument is also provided with a horizontal angle indicator that indicates the bearing of the EDM in a horizontal direction, i.e., the horizontal angle in relation to a reference angle position, and also with a vertical angle indicator that indicates the vertical angle of the EDM in relation to a horizontal plane. The instrument calculates the horizontal distance and the height differential on the basis of the EDM's measuring result and the signal given by the vertical angle detector. The measured values of horizontal length, horizontal angle and height difference are compared with setting-out point data input to the instrument before the measuring. Thus, preset target values for the horizontal and vertical angle indicator as well as the EDM are compared with actual values output by the horizontal and vertical angle indicator as well as the EDM. The result of this comparison is fed to an indicator, which produces an optical signal that can be readily discerned by the prism bearer and which has mutually different coding indicating whether and how the prism should be moved in order to be located from a starting point to the preset setting-out point different from the starting point. The setting-out point has been reached as soon as the preset values for the horizontal and vertical angle indicator as well as the EDM match with the actual output values by the horizontal and vertical angle indicator as well as the EDM. As an example it is proposed that this coding is such that the indicator produces a red light if the prism needs to be moved to the right and a green light if the prism needs to be moved to the left to be located at the next setting-out point. Alternatively it is proposed to transmit a Morse code. According to an example, the indicator emits two light beams of different colors (e.g. green and red) that are slightly divergent, such that the light beams overlap in a narrow central zone to indicate a measurement axis of the instrument to the user carrying the prism.
It is a disadvantage with this prior art that the tracking of the prism in the path of a measuring beam emitted from an electronic distance meter of the instrument is difficult. The instrument has to be tilted constantly about two orthogonal axes to keep the prism in alignment with the narrow measuring beam emitted from an electronic distance meter along the measurement axis. Thus, there is a high risk that the tracking fails and has to be repeated. This means that the user has to return to the instrument and to manually operate the instrument to bring the measuring beam into alignment with the prism before a new tracking can be performed. This is very time consuming if the distance between the instrument and the prism is large. Alternatively, the line-sighting instrument described with respect to U.S. Pat. No. 4,560,270 may be used to bring the prism in alignment with the measuring beam. In this case, the instrument known from U.S. Pat. No. 5,051,934 suffers from the same problems as the line-sighting instrument. Without this line-sighting instrument, it would be impossible to arrange the prism in the measuring beam emitted from the EDM to start the tracking procedure without manually aligning the instrument to the prism.